Method for the growth of semiconductor ribbons

ABSTRACT

The present invention provides a method for the continuous production of semiconductor ribbons by growth from a linear molten zone. The creation of the molten zone is achieved by application of an electric current, direct or alternating, parallel to the surface of the ribbon and perpendicular to the direction of growth, and intense enough to melt the said material, preferably using electrodes of the said material. The molten zone is fed by transference of the material, in the liquid state, from one or more reservoirs, where melting of the feedstock occurs. Preferably, the said electrodes and the said reservoir(s) are only constituted by the said material, thus avoiding contamination by foreign materials. The present invention is applicable, for example, in the industry of silicon ribbons production for photovoltaic application.

1. FIELD OF THE INVENTION

The present invention relates to a process for the continuous growth ofribbons of semiconductor material from a melt of the same material, andmore particularly to ribbon growth from a molten zone.

2. STATE OF THE ART

The crystallization of a semiconductor material into a ribbon, or foil,directly from the melt, is attractive because it eliminates the kerfloss and the cost associated to the operation of cutting an ingot intowafers. Several methods have been proposed for the continuous, or semicontinuous, growth of semiconductor ribbons. Few, however, have provenindustrial viability; there are two currently in production (derivedfrom U.S. Pat. Nos. 4,118,197 and 4,661,200).

Some of the problems are growth stability, high energy and consumablesuse, and low quality of the produced material, due in particular tocontamination by contact of the molten semiconductor with foreignmaterials, such as the ones used in crucibles, or in other furnaceelements, for instance ribbon shapers.

Some of the proposed methods avoid the use of a crucible bycrystallizing the ribbon from a molten zone (for instance, U.S. Pat. No.4,690,797); these, however, use the starting material in a pre-shapedform, or pre-ribbon, and the added step for obtaining this fromfeedstock makes them less attractive. Only for ingot growth have somepossibly crucibleless methods described continuous feeding of a melt (inthe form of a molten cap) directly with feedstock, such as cited byJP5043376, in 1993, and JP6199589, in 1994, where it was shown how togrow a silicon ingot from a molten cap fed with granular material orpowder, respectively.

Concerning the techniques used for heating a semiconductor material to amelt, Joule effect due to electrical currents in the material to becrystallised itself (both in the solid and molten phases) is among themost used. Most techniques use induced currents produced by an externalcoil; a few use electric currents injected by electrodes. Such is thecase of the method described in U.S. Pat. No. 3,960,511, in whichcurrent is applied transversely, along all the length of the ribbonforming melt, by electrodes acting also as shapers; and of thatdescribed in U.S. Pat. No. 3,935,058, where increased heat control isobtained in the melting zone of a zone melting process utilizing anelectric current passed longitudinally through the charge, to melt thecharge.

3. SUMMARY OF THE INVENTION

The present invention provides a method for the continuous production ofribbons of semiconductor material by growth from a linear molten zone.The creation of this molten zone, linear in the sense that it is verymuch longer than wide or thick, is achieved by application of anelectric current, direct or alternating, parallel to the surfaces of theribbon and perpendicular to the direction of growth, and intense enoughto melt the said material, preferably using electrodes of the saidmaterial. The molten zone is fed by transference of the said material,in the liquid state, from one or more reservoirs where melting of thefeedstock occurs. Preferably, the said reservoir, or reservoirs, areconstituted only by the said material, thus avoiding contamination ofthe melt by foreign materials.

The present invention is applicable, for example, in the production ofsilicon ribbons for the photovoltaic solar cell industry.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, schematic and simplified representation of anembodiment of the process in accordance with the present invention; and

FIG. 2 is a perspective, schematic and simplified representation ofanother embodiment of the process in accordance with the presentinvention.

5. DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the embodiment illustrated in FIG. 1, the ribbon of semiconductormaterial (1) is extracted from a linear molten zone (3) extending alongthe width of the supporting plate (2). The molten zone is created andkept using an electric current parallel to the surface of the ribbon (1)and perpendicular to the direction of growth of the said ribbon,indicated by the arrow in the drawing. The intensity of the saidelectric current must be high enough to melt the said material, thusproducing the said molten zone. Use of direct current is preferred,although alternating current can also be used.

Preferably, a system of radiative pre-heating can be used to assist theformation of the molten zone. The electric current, generated by thecurrent source (9), is applied through electrodes (4) and (5),preferably of the same material as the said ribbon.

A reservoir (6) of feedstock material in the liquid state is located inelectrode (4). The reservoir is fed by feedstock, preferably ingranular, or powder, form, that is continuously added to the reservoirby a guide (7). The reservoir (6) is heated, to keep a volume ofmaterial in the molten state, using preferably an induction coil (8). Inalternative, the feedstock can be molten by other means, such asradiation.

The molten zone (3) extends to connect with the reservoir (6), and thusthe said zone is fed by transference of the said molten material fromthe reservoir. Preferably, the said molten zone also extends out to theelectrode (5).

A second reservoir connected to the prolongation of the molten zone onthe electrode (5) can be used. This second reservoir, not shown in FIG.1, may act as a second source of material to feed the molten zone or mayact as a sink for extraction of existing impurities in the said moltenmaterial.

Preferably a pressure differential is applied between the reservoir ofmolten material (6) and the molten zone (3) in order to control the masstransference between the said reservoir and the said molten zone.

Preferably, elements (10) and (11) are used for stabilization of theedge of the ribbon and for conditioning mass transference. Theseelements are preferably located between the electrodes (4) and (5) andthe ribbon (1) and supporting plate (2). Preferably these elements areof the same material as the ribbon.

One other embodiment of the present invention is illustrated in FIG. 2.This process is characterized by the molten zone (3) havingsubstantially the form of a polygon.

The extracted material (1) has, in the particular case illustrated, theform of a hexagonal tube, which can later be cut longitudinally in orderto produce ribbons of semiconductor material. The molten zone (3) is inthis case created and kept by two electric currents, the first onegenerated by the current source (9) and applied by electrodes (4) and(5), and the second current generated by induction through inductioncoils (10). As previously, the molten zone is fed by a reservoir (6)where melting of the feedstock, continuously added to the reservoir by aguide (7), occurs. The material in the reservoir (6) is moltenpreferably using induction coils (8).

Having thus described a preferred embodiment of the present invention,it should be noted by those skilled in the art that the withindisclosures are exemplary only and that various other alternatives,adaptations and modifications may be made within the scope of thepresent invention. Accordingly, the present invention is not limited tothe specific embodiments as illustrated herein.

1. A process for the growth of ribbons of a semiconductor material froma long, thin molten zone, having a length similar to the width of thesaid ribbon, comprising: (a) providing semiconductor material in aliquid state in one or more reservoirs, in which a feedstock of the saidsemiconductor material is molten; (b) transferring molten semiconductormaterial from said one or more reservoirs to said molten zone by meansof a pressure differential applied between the said reservoir and thesaid molten zone; (c) the molten zone being a continuous film of liquid(i) suspended between a solid-liquid interface of the growing ribbon, onone side, and that of a supporting plate of the same material, on theopposite side, with two free lateral liquid-gas interfaces, so, that theliquid only contacts, along all its length, a solid phase of the saidsemiconductor material and no other substance, apart from the gas overits free surfaces, and (ii) created by an electric current,substantially parallel to the surface of the said ribbon andperpendicular to the growth direction of the said ribbon, passing in thesemiconductor material with an intensity required at least to form acontinuous suspended melt that constitutes the said molten zone.
 2. Aprocess in accordance with claim 1, in which the molten zone andadjacent solid semiconductor material, including the growing ribbon, areheated not only by an internal electric current but also by othercomplementary sources of heat, such as radiation, in order to achieveappropriate growth conditions.
 3. A process in accordance with claim 1,where in the said semiconductor material is silicon.
 4. A process inaccordance with claim 1, wherein the said reservoir(s) are constitutedof said semiconductor material.
 5. A process in accordance with claim 1,wherein the electrodes that introduce said current are constituted ofthe said semiconductor material.
 6. A process in accordance with claim1, comprising: using a plate shaped element(s) to stabilize the edge ofthe ribbon and conditioning the transference of the liquid to or fromthe said molten zone.
 7. A process in accordance with claim 6, in whichthe said element(s) are constituted of said semiconductor material.
 8. Aprocess in accordance with claim 1, wherein the said molten zone has theshape of an ellipse or a polygon.
 9. An apparatus for the growth of aribbon of semiconductor material from a molten zone comprising: a plateof semiconductor material; one or more reservoirs of said moltensemiconductor material; a pair of electrodes extending to the edges ofsaid plate; means connected to said electrodes for passing an electriccurrent to form a floating molten zone in said plate and to develop apressure differential capable of transferring said semiconductormaterial in a liquid state from said reservoirs to said floating moltenzone; and means to draw a ribbon of semiconductor material from saidfloating molten zone while the molten zone continues to be heated bysaid electric current and replenished by said semiconductor material inthe liquid state transferred from said reservoirs.